Foundations soil loading

While a full-scale field loading test, properly performed and interpreted, will reliably define the foundation soil interaction, such tests are generally not feasible. Thus, the way a soil or rock mass responds to being stressed is usually determined by previous experience in similar conditions, by extrapolating the results of small load tests or by using specific soil properties in various empirical formulae. These soil properties are sometimes inferred from previous experience, but more often reflect the results of laboratory and field tests on soil and rock samples. 

Two major considerations are required to design a foundation for a self-supporting tower soil loading and tower stability. The load on the soil below the tower must not exceed the maximum load the soil will support. The foundation must keep the tower in a vertical position so it will not be overturned by the maximum forces acting on it. 

Having calculated Si and S2 it is possible to calculate the soil loading, S, which is the sum of the two. The maximum soil load occurs at the edge of the foundation shown as point F in Figure 11-1. Many structural engineers call this the soil pressure. 

The poorest stability occurs just before the tower is placed on the foundation. To calculate the tower s stability. Si must be replaced with Si , which is the minimum soil loading caused by the dead load, psf. 

The value of e calculated from equation (11-11) is maximum for any particular foundation, which is the value governing stability. We are now concerned with the maximum soil loading (toe pressure) which occurs when the dead load is maximum. It is therefore necessary to substitute W in place of Wf in equation (11-11) ... 

It was shown by equation (11-1) that the total soil loading, to be considered in the design of tower foundations, is the sum of Si, the dead load, and S2, the load caused by the overturning or wind moment. There is no overturning moment on guyed towers however, the wind pressure does have an important effect on the foundation, as the soil is required to resist the vertical component of the pull on the guy wires. 

When the safe soil loading is very low, it is sometimes difficult to design an ordinary foundation... 

Having selected a foundation of such size and shape as to fulfill the requirements of the problem from the standpoint of stability and soil loading, it becomes necessary to calculate the stresses in the foundation itself, to see that they do not exceed the allowable limits. 

Reinforcement to Resist Uplift Stresses. The wind moment creates a positive soil load on one side of the centerline, and a negative load on the opposite side. In other words, the action of the wind tends to lift the foundation on the negative side. This upward force, or uplift effect, is resisted by the weight of the concrete base itself, and by the weight of the earth fill on top of the base. It, therefore, becomes necessary to reinforce the top of the base to resist the resulting negative bending moment. 

The purpose of foundations is to distribute the loading from structures and equipment so that perpetual settlement of the load-bearing soil will not cause excessive maintenance or impair the usefulness of the plant. The selection of a suitable type of foundation or soil loading support structure depends on the loads to be transferred to the foundation, on the material on which the foundation rests, and on the method of placing the foundation as dictated by the subsoil conditions. 

When subjected to maximum wave and wind loads the structure shall have an acceptable factor of safety against horizontal sliding and against a shear failure in the foundation soil. 

Owing to shear stresses induced in the foundation soils by the vertical eccentric foxm-dation load, the magnitude of horizontal load that the foundation soils can sustain may decrease witii an increase in vertical load. This fact must be recognized when computing and evaluating sliding resistance by the simplified procedures described below. 

Verification of the soil strength should not, however, neglect the foundation soil bearing capacity for higher loads caused by an earthquake, the resistance of slopes, soil support walls or of other works of interest for safety, also considering potentially induced indirect effects, such as flood waves in streams due to the failure of dams (Hansen, 1970 Meyerhof, 1951 Janbu, 1957 Morgenstern and Price, 1965 Sarma, 1975, 1981 Espinoza, Bourdeau and Muhunthan, 1994). 

The capacity of the foundation soils to bear the dynamic loads transmitted by the structure is verified when the ratio between the load acting on the foundation and glim is higher or equal to 1 but which includes a safety margin (e.g. 1.2). 

The response spectrum of the aircraft impact pulse is rather hard , that is dominated by high frequencies. For this reason, the components subjected to the highest loads are the most rigid ones, especially if the plant is located on rigid foundation soil (rock). 

Earth dams are usually constructed on clay soils as they have insufficient load-bearing properties required to support concrete dams. Beneath valley floors, clays may be contorted, fractured and softened due to valley bulging so that the load of an earth dam may have to spread over wider areas than is the case with shales and mudstones. Settlement beneath an embankment dam constructed on soft clay soils can present problems and may lead to the development of excess pore water pressures in the foundation soils (Olson, 1998). Rigid ancillary structures necessitate spread footings or raft foundations. Deep cuts involve problems of rebound if the weight of removed material exceeds that of the structure. Slope stability problems also arise, with rotational slides being a hazard. 

Reinforcement in a slope contributes to stability in two ways. Firstly, the reinforcement directly improves the shear resistance of the soil to resist the shear loading caused by the steep face. Secondly, the reinforced zone acts to hold the unreinforced soil mass of the interior in equilibrium without overstressing the underlying foundation soils. 

Mohammed AMY, Okhovat MR, Maekawa K (2012b) Nimierical investigation on damage evoluti(m of piles inside liquefied soil foundation-dynamic-loading experiments. Int J World Acad Sci Eng Technol 71 1796-1803... 

Consolidation tests are commonly performed to (1) evaluate the compressibOity of soil samples for the calculation of foundation settlement (2) investigate the stress history of the soils at the boring locations to calculate settlement as weU as to select stress paths to perform most advanced strength tests (3) evaluate elastic properties from measured bulk modulus values and (4) evaluate the time rate of settlement. Consohdation test procedures also can be modified to evaluate if foundation soils are susceptible to coUapse or expansion, and to measure expansion pressures under various levels of confinement. Consohdation tests include incremental consohdation tests (which are performed at a number of discrete loads) and constant rate of strain (CRS) tests where load levels are constantly increased or decreased. CRS tests can generally be performed relatively quickly and provide a continuous stress-strain curve, but require more sophisticated equipment. 

The rate of the load application and the permeability of the foundation geomaterials need to be considered in the foundation design, especially when classifying a certain soil as either cohesive or cohesionless. For the purpose of foundation design, it is more appropriate and useful to classify foundation soils as either coarse-grained or fine-grained compared to the often used cohesionless or cohesive, respectively. 

Soil behavior under applied loads is significantly affected by the presence of water in the pores or voids, and how quickly relative to the rate of the load application the pore water can flow out of the stressed foundation soil zone. Soil, when subjected to loads, tends to change in volume. For this volume change to occur in saturated soils, the pore water that is considered relatively incompressible compared to the soil skeleton needs to flow out of the stressed soil zone. 

When shallow foundations are constructed on, within, or near slopes or recently placed fill embankments and adjacent to another structure, potential exists for additional settlement or deformation occurring beyond the directly stressed or foundation zones or due to loads other than those imposed by the foundation under consideration. For example, settlement of the soils below the foundation level due to the additional loads imposed by recently placed embankment fill at the abutments will add to the foundation settlement due to applied structure service loads. Furthermore, a certain type of foundations soils, known as collapsible soils, may experience significant compression due to the introduction of additional moisture. 

The above discussion on foundation settlement is equally applicable to upward movement of foundations. Upward movement of foundations can be due to heave of the foundation soil or upUft of the foundation. Heave of shallow foundations occurs due to increase in the volume of expansive soils when additional moisture is introduced into these soils. Heave usually is only a concern for lightly loaded foundations and not for most bridges, because their foundations are generally heavily loaded. Uplift movements of foundations occur due to the externally applied upward (or tensile) load on foundations. Bridge foundations, in particular shallow foundations, are not usually allowed to be subjected to service level or sustained uplift forces. Thus, uplift movement is generally not a design concern for shallow foundation used to support bridge structures. 

For a given footing size, the settlement at which the apphed uniform contact stress (qj becomes equal to the ultimate unit bearing capacity (q ,) of the footing, as defined earlier, depends on the types and conditions of the foundation soils and the rate of loading. 

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